dynamic category sidebar

function abc_show_sidebar() {
    // default value
    $sidebar_id = 'sidebar';

    // first check to see if this is a category page
    if ( is_cat() ) {
        $sidebar_id = sanitize_title( get_cat_name( get_query_var( 'cat' ) ) ) . '-sidebar';
    }
    // now check if this is a single post page
    elseif ( is_singular( 'post' ) ) {
        // grab the $post object
        $post = get_post();
        // here is where you have to make the assumption that the post
        // has only one category assigned to it
        // get_the_category() returns an array of cat objects assigned to the post
        $categories = get_the_category( $post->ID );
        // use only the first
        $category = $categories[0];
        // this will go up through the category tree to find the top-level parent
        while ( 0 != $category->parent ) {
            $category = get_category( $category->parent );
        }
        $sidebar_id = $category->cat_name;
    }

    dynamic_sidebar( $sidebar_id );

}